Comparative Transcriptome Analysis Reveals The Molecular Mechanisms Of Fiber Length And Vegetative Growth Heterosis Between Gossypium.hirsutum.and G.barbadense.

To study the hereditary roles of fiber length and vegetative growth heterosis of upland-island hybrids,in this study,53 upland-island hybrid combinations were prepared by using 12 upland cotton varieties and 5 island cotton varieties mainly cultivated in China.The fiber quality traits of parents and F₁were planting in Sanya and Lin’an,and analysis these combination’s heterosis.Then,a special combination（Luyuan343×Achang599）with significant over-parent heterosis（OPH）was selected as material to determine and analyze the characters related to fiber development and vegetative growth at seedling stage.On this basis,ovules,fibers and leaves at different developmental stages were used as materials.Transcriptome analysis and related enrichment pathway were carried out.The results are as follows:Fiber quality data showed that fiber length were significant mid-parent heterosis（MPH）in all hybrid combinations,and some hybrid combinations had strong over-parent heterosis.In addition to T003,T009 and T039,fiber strength had significant MPH too,and the micronaire of more than 60%hybrid combinations had significant negative MPH.The uniformity index of more than 50%hybrids had significant positive MPH,and only 40%of the combinations had significant positive MPH.For11 hybrids with fiber quality above 4A grade and replanted in Lin’an,Zhejiang Province.The fiber length and strength decreased slightly compared with Sanya,but still remained above 4A grade,and showed significant MPH.T027 still maintained the most significant OPH.After measuring the immature fiber length of the most significant heterosis combination（T027）,it was found that the difference of fiber length heterosis was the most significant within 10 days post anthesis（DPA）,and the heterosis tended to be flat from 15 to 20 days post anthesis.RNA-seq and Small RNA-seq of 0 DPA and 5 DPA’s ovules and 10 DPA and 15 DPA’s immature fibers showed that10 DPA was the period with the largest number of differentially expressed genes（DEG）,and the DEG from upland cotton were significantly more than those from island cotton,and were mainly enriched in auxin signal response pathway in plant hormone signal transduction.After enriching the target genes of differentially expressed mi RNA,it was found that the target genes were mainly enriched in the auxin related pathway,in which Ghr-mi R393b negatively regulated the target gene ARF18,and then regulated the auxin output signal,which may be the reasons for heterosis.Through the determination of IAA content,it was found that the auxin content in different materials decreased with the passage of time,but the auxin content in F₁was higher than that of upland cotton and island cotton parents in all periods.It may be due to the change of auxin content leading to heterosis.The results of ovule culture in vitro showed that the fiber initiation of F₁generation with island cotton as female parent was earlier than that of upland cotton,and the growth of reciprocal fiber in F₁generation was better than that of control after adding auxin export inhibitor,which verified the results of transcriptome and mi RNA analysis.At the same time,the related genes were verified by q-RT PCR,which was consistent with the results of RNA-seq.According to the study of inverted one leaf in cotyledon stage,one leaf stage and three leaf stage,it was found that 15 days after sowing was the most significant period of reciprocal biomass heterosis of land and sea hybrid cotton,and the significant difference gradually decreased after 23 days of sowing.The analysis of transcriptome data showed that the differential genes were enriched in photosynthesis,cell components,chloroplast and other pathways.At the same time,differentially expressed genes also enriched in cytoplasm and cytoplasm,which may be due to maternal effect.In this study,a number of field data of hybrids and parents,transcriptome data and mi RNA data of seedling,ovule and immature fiber were obtained,which revealed the DEG between upland-island hybrids and their parents at the transcriptional level,which provided a valuable theoretical basis for exploring the molecular mechanism of fiber length and vegetative growth heterosis.